Bolt assembly for firearm



Jan. 16, M. A. SMITH BOLT ASSEMBLY FOR FIREARM Filed March 30, 1967 IN VEN TOR. %MJM Yum.

/4 7 TOR/W75 United States Patent Ofiice 3,363,351 Patented Jan. 16, 1968 3,363,351 BOLT ASSEMBLY FOR FIREARM Melvin A. Smith, 280 N. Ngain St., Wilkes Barre, Pa. 187 2 Filed Mar. 30, 1967, Ser. No. 626,994 8 Claims. (Cl. 4Z16) ABSTRACT OF THE DISCLOSURE An automatic or semi-automatic firearm having a bolt assembly wherein the bolt is provided with a pair of longitudinally disposed cylindrical bores, each of which has positioned therein a hexagonally shaped guide rod which supports the bolt for slidable movement thereon, the bolt thereby having a reduced tendency to jam due to the accumulation of foreign matter on the surfaces of the bolt assembly.

This invention relates to an improvement in firearms of the automatic and semi-automatic type and more specifically to an improvement in the bolt assembly of such firearms.

Automatic and semi-automatic firearms having a bolt mounted for to and fro movement on a pair of parallel, cylindrically shaped guide rods positioned in the receiver of the firearm are known. A bolt of this type is provided with a pair of parallel, axially disposed cylindrical bores, each of the bores receiving one of the aforementioned pair of guide rods. Additionally, each of the guide rods is provided with a recoil spring which cooperates with the trigger mechanism of the firearm to urge the bolt from a cocked position to a firing position as the weapon is being fired. During this movement, the bolt generally cooperates with other parts of the firearm to strip a cartridge from the magazine, position the cartridge for firing and to fire the cartridge. Upon being fired, the expanding gases of the exploding cartridge drive the bolt against the recoil springs to its cocked position. The energy in the springs when the firearm is triggered causes the cycle to repeat by urging the bolt once again to its firing position. The guide rods thus serve as a mount or support on which the bolt reciprocates during the firing of the weapon.

A bolt having cylindrical bores and being slidably mounted on a pair of cylindrically shaped guide rods, as discussed above, has various economic and functional advantages over other types of bolt assemblies, including, for example, bolt assemblies wherein the bolt is slida-bly mounted on guide supports which are an integral part of the internal Walls of the receiver. The manufacture of the last mentioned type of assembly requires that more intricate and close tolerance machining operations be performed on the receiver walls and the bolt in order to form the guide support on the receiver walls and the complementary mounting means on the bolt. In addition, the walls of the receiver must be relatively thick or of heavy construction in order to be suitable for the forming of such guide support and to withstand the reciprocating action of the bolt. This adds to the cost of manufacture and materials and increases the Weight of the weapon. On the other hand, the manufacturing and materials expenses of a bolt assembly of the type having the bolt supported on guide rods are not as great. Because the bolt is supported independently of the receiver walls, the receiver can be formed by a relatively inexpensive stamping operation or a similar operation. For the same reason, the receiver can be of less heavy construction thereby reducing the overall weight of the weapon.

However, as is well known, there is a tendency for small particles of foreign matter to accumulate on the surfaces of reciprocating bolt assemblies, in consequence of which repeated cleaning is required in order to prevent slowing down of the bolt action or jamming. This is true of bolt assemblies of various prior types including those in which the bolt is mounted for reciprocation on guide rods, and even more so with assemblies in which the reciprocation of the bolt is guided by the receiver walls. As mentioned above, the bolt during its movement from the cocked position to the firing position cooperates with other parts of the firearm to feed and fire the cartridges. It has been found that as the Weapon is used over a period of time, the velocity of the reciprocating bolt tends to decrease to such an extent that it is unable to strip, feed and fire the next round of ammunition. This results in a stoppage of firing of the weapon.

The cause of the slowdown of the velocity of the bolt has been traced to small particles of foreign matter, which initially appear as black smudges on the surfaces of the bolt assembly, and which tend to eventually accumulate as deposits or layers of suffcient thickness to create frictional forces which interfere with the 'free movement of the bolt. This foreign matter tends to become trapped in the clearance between the bolt and the bolt support surfaces. For example, in a bolt assembly of the type having the bolt supported on guide rods, the foreign matter tends to accumulate in the clearance between the cylindrically shaped guide rod and the surface of the cylindrical bore, such clearance, for example, being about 0.002" to about 0.004". As the deposits build up in thickness, the resulting increase in frictional forces cause the bolts velocity to be decreased to such an extent that the bolt does not have sufiicient momentum to strip and fire the next cartridge.

The sources of the foreign matter which cause this problem are powder residue from the fired cartridges, and dirt, dust, fine sand, etc. from the environment in which the firearm is used. The accumulation of foreign matter and the resultant slowdown of the bolt action and tendency to jam is thus aggravated by certain use factors, such as the type of ammunition employed and the circumstances under which the weapon is being fired. Frequently the locale of the use of military weapons is such as to greatly aggravate the accumulation of dirt, with the result that the weapon is likely to jam at the very time when is use is a matter of life and death. Moreover, under such conditions, frequent cleanings are not practical.

It is therefore an object of this invention to provide an improved bolt assembly for a firearm of the automatic or semi-automatic type characterized by having a strikingly reduced tendency to jam as a result of the accumulation of foreign matter between the bolt and its supporting surfaces.

In accordance with this invention, the above object is obtained with a bolt assembly of the type having the bolt supported on guide rods by providing the guide rod with at least two spaced bearing surfaces with a clearance surfact therebetween. More specifically, it has been found that if the cross-sectional shape of the guide rod and of the bore in the bolt are inter-related so that a clearance space is provided, the tendency of the bolt to malfunction as described hereinabove is drastically decreased. The clearance space accommodates the accumulation of a considerable amount of foreign matter which has little or no tendency to interfere wit-h the reciprocating action of the bolt.

In the preferred embodiment, the bore in the bolt is cylindrical in shape and the shape of the guide rod is such that it has a multiplicity of spaced bearing surfaces guide and support the bolt as it moves to and fro on the guide rod. The faces of the polygonally shaped guide rod are the clearance surfaces which are in spaced rela' tion to the interior of the bore. Thus in the preferred form of this invention, the shape of the guide rod is such that there is provided a multiplicity of bearing surfaces alternating with a multiplicity of clearance surfaces. A hexagonally shaped guide rod is particularly suitable for these purposes.

The operational improvement of a firearm equipped with a bolt assembly in accordance with this invention is illustrated by the following example which compares the firing of a weapon equipped initially with a convcntional bolt assembly and then with a bolt assembly in accordance with this invention.

A firearm generally known as a carbine of caliber .45 ACP, capable of firing about 426 rounds of ammunition per minute, was utilized in the comparative test, This firearm was provided with a conventional bolt assembly, that is, one wherein the bolt was slidably mounted on a pair of cylindrically shaped guide rods each of which are positioned in a cylindrical bore of the bolt, and was continously fired until about 875 rounds of ammunition were expended. At this time the bolt failed to return to its firing position with sutficient velocity to feed and fire the next round of ammunition. The firearm thus was in firing operation for somewhat over two minutes before the malfunctioning occurred. On the other hand, the same firearm equipped with a bolt assembly of this invention, that is, one wherein the bolt was slidably mounted on a pair of hexagonally shaped guide rods each of which was positioned in a cylindrical bore of the bolt, was fired without failure until about 8600 rounds of ammunition were expended. Firing of the weapon was stopped at this time because of a lack of additional ammunition and not because of malfunctioning of the bolt assembly. The

carbine thus was in firing operation for somewhat over minutes without malfunctioning occurring.

The above comparative test clearly shows the outstanding and surprising operational improvement of a firearm equipped with a bolt assembly of this invention. The number of rounds of ammunition fired and the firing time was almost ten times greater than that of the firearm equipped with the conventional bit assembly.

A preferred embodiment of this invention is described hereinafter in detail in connection with the accompanying drawings wherein:

FIGURE 1 is a side view of a firearm of a type to which this invention is applicable;

FIGURE 2 is a fragmentary side view, partly in section, on an enlarged scale as compared with FIGURE 1 showing the bolt assembly of this invention housed in the receiver of the firearm;

FIGURE 3 is an isometric view of the bolt assembly on an enlarged scale as compared with FIGURE 2;

FIGURE 4 is a sectional view taken on the line 4-4 of FIGURE 3 and on an enlarged scale as compared with FIGURE 3;

FIGURE 5 is a fragmentary sectional view on the line 55 of FIGURE 2 and on an enlarged scale as compared with FIGURE 2; and

FIGURE 6 is a fragmentary view similar to FIGURE 4 showing another embodiment of this invention.

Referring now to FIGURE 1, there is shown a firearm generally referred to as a semiautomatic carbine or sub Lil machine gun having a receiver 2, a butt stock 4, a trigger housing 6, a magazine 8, a cartridge ejector port 10, a foregrip 12, and a barrel 14.

In FIGURE 2, there is shown a preferred embodiment of the bolt assembly 16 of this invention housed in the receiver 2.

The bolt assembly 16 is more clearly shown in FIG- URE 3. A pair of parallel, hexagonally shaped guide rods 18 and Z0 is fixed to the guide rod retainer plug 22 by means of a pin 24 positioned in the pin receiving hole 26 of the guide rod retainer plug 22. The guide rods may be made of any suitable metal and are preferably made of steel, which, for example, has been case hardened or surface hardened as by hard chrome plating.

The bolt 28 is comprised of the bolt body 3th and the bolt extension 32 which is an integral part of the bolt body. The bolt extension generally is provided with a cartridge extractor and an integral firing pin, neither of which is shown.

The bolt body 3t is provided with a pair of axially disposed cylindrical bores, 34 and 36 extending the length of the bolt body. As seen in FIGURE 5, each of the bolts cylindrical bores 34- and 36 is of stepped construction, with the rear section of the bore being of larger diameter than the forward section of the bore. The positioning of the bolt extension 32 and the bores 34 and 36 in the bolt body 30 is such that when the fired cartridge is ejected from the firearm, it will fly past the guide rods 18 and 20, out of the ejector port 10 and away from the shooter of the firearm.

The bolt 28 is slidably mounted on the guide rods 18 and it} on which are positioned recoil springs 38 and 4-0 respectively. One end of each of the recoil springs abuts against the guide rod retainer plug 22 and the other end of each of the springs abuts against one of the shoulders 42 and 44 in the cylindrical bores 34 and 36.

FIGURE 2 clearly shows the bolt assembly 16 positioned in the receiver 2 of the firearm. The guide rod retainer plug 22 is held in a fixed position in the receiver 2 by the screw 46. The receiver front plug 48 is held in a fixed position in the receiver 2 by brazing to the receiver. The receiver front plug 48 serves as th support for the front ends of the guide rods 18 and 20, each of which fits snuggly in one of the holes 50 and 52, as more clearly seen in FIGURE 3. The receiver front plug 48 also serves as the support for the barrel 14.

With reference now to FIGURE 4, it will be seen that the edges 18' of the guide rod 13 and the edges 20' of the guide rod 20 are in line contact with the respective bolts internal surfaces which form the cylindrical bores 34 and 36. These edges 18' and 20 are the bearing surfaces on which the bolt is slidably supported. Because the edges 13' and 20 adequately support the bolt, the lowermost edges of the guide rods need not be in contact with the bolts surfaces which form the cylindrical bores (as shown) and thus it is unnecessary in the manufacture of the bores of the bolt and the guide rods to provide these parts with such close tolerances that each of the edges of the guide rods would be in contact with said surfaces. This could be done if desired, but there is no advantage in doing so.

Still with reference to FIGURE 4, the faces 18 of the hexagonally shaped guide rod 18 and the corresponding faces 2%" of the guide rod 20 are the clearance surfaces of the guide rods and are not in contact with the surfaces of their corresponding bores 34 and 36, thus providing clearance spaces between the surface of the guide rods and the surfaces of the bores. It can be thus seen that the bearing surfaces and clearance surfaces of each of the guide rods cooperate to support and guide the bolt while at the same time, there are provided clearance spaces which will allow the accumulation of relatively large amounts of foreign matter on the surfaces of the guide rods and the surfaces of the bores, which foreign matter will not interfere with the reciprocating motion of the bolt on the guide rods.

In its preferred embodiment, the bearing surfaces of the guide rod are edges which are in line contact with the bore in the bolt. This provides for relatively small friction as the bolt slides on the guide rods and also maximum clearance spaces.

Although it is preferred that the bearing surface of the guide rod be a line contact as described above, the invention is also applicable to embodiments wherein the bearing surface is in true surface to surface contact with the surface of the guide rod receiving hole. Such embodiments are not as effective as ones wherein the bearing surfaces are in line contact with the surface of the guide rod receiving hole, but are an improvement over a cylin drically shaped guide rod positioned in a cylindrical bore of the bolt. An example of such embodiment is illustrated in FIGURE 6 which shows the cross-sectional shape of a guide rod 60 in a guide rod receiving hole 62 of the b0lt 64. Thus, when the term bearing surface is used herein, it includes within its scope bearing surfaces which are in line contact with the guide rod receiving surface and also bearing surfaces which are in true surface to surface contact with the guide rod receiving surface.

This invention provides an improved bolt assembly which has much less of a tendency to malfunction during continued operation of the firearm than heretofore known bolt assemblies. The invention is directed to the finding that if the cross-sectional shape of the guide rod and the cross-sectional shape of the guide rod receiving hole of the bolt are interrelated so that a clearance space is provided between the surface of the guide rod and the surface of the bore, the tendency of the bolt to malfunction due to the accumulation of deposits of foreign matter on critical surfaces of the "bolt assembly is drastically decreased. The interrelated cross-sectional shapes can be provided in a variety of ways other than shown. For example, the bolt can be provided with an angular bore rather than a cylindrical bore and the guide rods can be cylindrical in shape. The bearing surfaces and clearance surfaces can be provided by striating either the guide rod surface or the surface of the guide rod receiving hole with longitudinal grooves.

I claim:

1. A bolt assembly for use in the receiver of a firearm of the semi-automatic or automatic type, the assembly comprising a bolt having longitudinal bore means therein and guide rod means positioned in the bore means of the bolt, the bolt being slidable to and fro on the guide rod means, the guide rod means having at least two spaced bearing surfaces with a clearance surface therebetween, the bearing surfaces bearing against the internal surface of the bore means, the cross sectional shape of the guide rod means and the bore means being so interrelated that at least one of the bearing surfaces is adapted for line contact with the bore means and the clearance surface being in spaced relation to the internal surface of the bore means.

2. A bolt assembly according to claim 1 wherein the cross-sectional shape of the bore means is circular and wherein at least one of the guide rod means has a multiplicity of bearing surfaces which are in line contact with the surface of the cylindrical bore means and a clearance surface between each of the bearing surfaces.

3. A bolt assembly according to claim 2 wherein the cross-sectional shape of at least one of the guide rod means is polygonal, the edges of the polygonally shaped guide rod means comprising bearing surfaces, and the faces of the polygonally shaped guide rod means comprising clearance surfaces.

4. A bolt assembly according to claim 3 wherein the cross-sectional shape of at least one of the guide rod means is hexagonal.

5. A bolt assembly for use in the receiver of a firearm of the semi-automatic or automatic type, the assembly comprising a bolt having a pair of longitudinally disposed parallel internal bores, each of the bores being of stepped construction with a shoulder between the bore part of larger diameter and the bore part of smaller diameter, a pair of parallel guide rods, each guide rod being positioned in one of the cylindrical bores of the bolt, the cross-sectional shape of each of the guide rods being polygonal, the edges of the polygonally shaped guide rods comprising bearing surfaces on which the bolt can slide to and fro, the faces of the polygonally shaped guide rods comprising clearance surfaces in spaced relation to the internal surfaces of the bores, mounting means for the guide rods and recoil springs on each of the guide rods, one end of each of the recoil springs abutting against the mounting means and the other end of each of the recoil springs abutting against a shoulder in one of the bores.

6. A bolt guiding rod assembly for use in a firearm of the semi-automatic or automatic type having a bolt provided with longitudinally disposed cylindrical guiding bore means, the guiding rod assembly comprising guide rod means adapted to be received in the cylindrical guiding bore means of the bolt, the guide rod means having at least two spaced bearing surfaces, at least one of the bearing surfaces being a bearing edge, with clearance surfaces as the adjacent sides of the edge, the bearing surfaces being adapted for slidable reception in the cylindrical bore means of the bolt for supporting and guiding the bolt, and the clearance surface providing a clearance space be tween the cylindrical bore means and the guide rod means, and the assembly further including support means for positioning the guide rod means in the receiver of the firearm in spaced relation to the internal walls of the receiver.

7. A bolt guiding rod assembly according to claim 6 wherein the guide rod means includes at least two guide rods positioned on the support means, each of the guide rods being in spaced relation and parallel to each other, and the cross-sectional shape of each of the guide rods being polygonal.

8. A firearm of the semi-automatic or automatic type including in combination a receiver and a bolt assembly positioned in the receiver, the bolt assembly comprising a bolt slidable to and fro in the receiver, the bolt having longitudinally disposed bore means, guide rod means positioned in the bore means, the bolt being slidable to and fro on the guide rod means, the guide rod means having at least two bearing surfaces with a clearance surface therebetween, the bearing surfaces bearing against the internal surface of the bore means, the cross sectional shape of the guide rod means and the bore means being so interrelated that at least one of the bearing surfaces is adapted for line contact with the bore means and the clearance surface being in spaced relation to the internal surface of the bore means, and supporting means for positioning the bolt assembly in the receiver so that the bolt is in spaced relation to the internal walls of the receiver.

References Cited UNITED STATES PATENTS 2,403,306 7/1946 Sampson et al. 42-l6 2,960,917 11/1960 Lizza 42l6 2,425,684 8/1947 Patchett 42l6 3,318,192 5/1967 Miller et al. 89-142 BENJAMIN A. BORCHELT, Primary Examiner.

T. H. WEBB, Assistant Examiner. 

